In regard to a formed article represented by a fastening component, such as a bolt and a screw, used in various products extending from a precision large machine such as automobiles and air craft to a small precision equipment such as various electric appliances and information equipment, a greater toughness and a greater increase in life are demanded from viewpoints of safety, convenience and compactification of the product, or the like. Moreover, about a manufacturing process of the formed article, it is also an important requirement to reduce a load to environment.
Hitherto, in regard to the toughening of a steel-made component, it is achieved by optimizing mainly an alloy constituent, and conditions of a working and a heat treatment. In regard to this, recently, there is studied a manufacturing technique of the formed article, in which an energy conservation, a conservation of natural resources, and a reduction in environment load become possible. For example, in JP-A-2004-60046 (hereafter, referred to as Patent Document 1), there are proposed a high strength formed article including a screw and the bolt, and a manufacturing method of the same, i.e., the high strength formed article whose strength is ensured by forming a hyperfine structure, e.g., a pressure-produced article, and various components and members, without depending on an addition of alloy element or a refining treatment, and the method capable of simply manufacturing that high strength formed article. That is, according to the Patent Document 1, there is disclosed the high strength formed article in which a wire rod whose mean grain diameter in short diameter is 0.5 μm in minimum is header-formed, and which is worked to a small screw or the bolt or a rivet by being form-rolled (refer to, e.g., paragraph number 0018, Table 2, Table 3 in the same Patent Document).
Patent Document 1: JP-A-2004-60046 However, in the same Patent Document, the mean grain diameter of the formed article is 0.5 μm even in minimum, and a tensile strength TS is 843 MPa even in maximum. And, especially, about a structure form and a strength as to a cold pressure-produced part in a head part of the wire rod made hyperfine grains to 0.5 μm in the mean grain diameter, there is neither description nor suggestion. Accordingly, in regard to the formed article in which a strength in a site, to which the cold pressure production is applied, is especially emphasized, and a strength higher than a level disclosed in the same Patent Document is demanded in a formed article main body, it is impossible to cope with it.